It has heretofore been well known that a composition comprising a polyorganosiloxane as a main component is cured on the surface of a substrate such as various kinds of paper, synthetic film and fiber to form a film, thereby imparting to the substrate releasability with respect to an adhesive substance.
A composition which is cured upon condensation reaction as disclosed in, for example, JP-A-47-34447 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and a composition which is cured upon addition reaction as disclosed in, for example, JP-B-52-40918 (The term "JP-B" as used herein means an "examined Japanese patent publication") are known as such polyorganosiloxane compositions. However, since these curing reactions require heating, these polyorganosiloxane compositions cannot be applied unless the substrate is heat-resistant.
In order to overcome the above-described difficulty of the thermosetting silicone compositions, various ultraviolet-curable silicone compositions have been developed as disclosed in, for example, JP-A-56-166224, JP-A-58-213024, JP-A-60-47064, JP-A-60-84329, JP-A-61-293268, JP-A-1-297421, and JP-A-1-311103. An ultraviolet-light emitter is one type of light emitter. Because of its inexpensiveness, easy maintenance and little danger to the operator, this emitter finds the widest application. A curing method based on ultraviolet light is generally advantageous in that it requires a short curing time. Such a curing method is also advantageous in that a substrate which is damaged by heat energy does not need to be heated after coating the silicone composition because the silicone composition can be cured by irradiation with ultraviolet rays.
The ultraviolet-curable silicone compositions are roughly divided according to cure mechanism into the following four types:
(1) Compositions which are cured by reacting an Si--Vi group (Vi stands for vinyl group) and an Si--H group in the presence of a platinum catalyst by means of ultraviolet rays; PA0 (2) Compositions in which an acrylic-functional silicone is cured in the presence of a radical cleavage-type photocatalyst by means of ultraviolet rays; PA0 (3) Compositions which are cured by reacting an Si--Vi group and an SH group in the presence of a radical cleavage-type photocatalyst by means of ultraviolet rays; and PA0 (4) Compositions in which an epoxy-functional silicone is cured in the presence of a cation-generating catalyst by means of ultraviolet rays.
Silicone compositions of type (1) above are economically disadvantageous in that the expensive catalyst must be used in a large quantity.
Compositions of type (2) cure quickly, but the curing reaction must be conducted in an inert gas atmosphere because the cure is inhibited by oxygen. For this reason, they are disadvantageous in that the apparatus therefor must be specially designed and the running cost is high due to the use of an inert gas.
Compositions of type (3) have excellent curability with little curing inhibition by oxygen. However, the compositions have disadvantages in that because they contain mercapto groups, they have a strong offensive odor, which is unfavorable to the workers, and in that the compositions are so unstable that their shelf lives are short.
Compositions of type (4) cure by means of ultraviolet rays without suffering curing inhibition by oxygen and do not emit an offensive odor. Thus, the compositions of this type have exceedingly good properties.
Further, the epoxy-functional silicone compositions of type (4) are advantageous in that they exhibit a higher adhesiveness to the substrate than that of other ultraviolet-curable silicone compositions.
However, these silicone compositions are disadvantageous in that they exhibit different releasabilities to different adhesives. In particular, they are poor in releasability to reactive acrylic adhesives.